The present invention relates to a method for casting objects, and more particularly, a method for casting railroad wheels using an improved riser arrangement.
The preferred method for manufacturing cast steel railroad wheels is a bottom pressure casting foundry operation wherein molten steel under pressure is forced upwardly into a machined graphite mold. The mold is thereby filled with molten steel from the bottom upwardly. This bottom pressure casting operation eliminates many of the concerns associated with traditional top pouring of molten steel into molds in foundry operations such as splashing and insufficient filling.
In the bottom pressure casting of railroad wheels, upon separation of the top half or cope of the mold from the bottom half or drag of the mold, it is usual for the top facing front side of the wheel to have, depending on the diameter size of the wheel, from 6 to 14 raised sections or risers extending upwardly from the plate portion of the wheel. These risers extending upwardly from the plate of the wheel near the rim are designed to hold additional metal deemed necessary to fill downwardly into the mold during the cooling and solidification of the wheel just after pouring. There are accepted standards for porosity of steel railroad wheels that must be met by designing an adequate number of risers and, accordingly, an adequate volume of metal left to fill downwardly into the molds during cooling and solidification of the wheel.
In a machined graphite mold, the graphite absorbs heat from the molten steel in a manner such that the molten wheel is fairly rapidly cooled and solidified at the outer surface in contact with the graphite. This allows a high production rate of wheels as the cope and drag can be fairly quickly separated from each other shortly after pouring thereby allowing the wheel to be properly cooled and otherwise heat treated during its manufacture. Due to the rapid absorption of heat from the molten steel by the graphite mold, it is current practice to line the risers that are machined in the cope section of the mold with a phenolic resin sand mixture. This mixture is premixed using a batch mixing operation that requires the sand to be heated to dissolve the resin. Such resin sand mixture is poured into the risers such that each riser is filled. The cope section of the mold has been preheated, and a curing heating pad is utilized to provide heating of the resin sand filling the risers of the cope. The heat cures a layer of sand such that each riser is coated with a liner of sand. This riser lining operation may be repeated if necessary. The excess sand is then poured out leaving a hollow riser with a sprue opening in the bottom at the wheel interface.
In current riser practice utilizing silica sand and heat setting phenolic resins, the liner of each riser leaves a layer of sand of a thickness of about 0.375 inches. The remaining diameter of each lined riser opening is about 4.250 inches, with an average height of about 7.5 inches. However, it is difficult to accurately control the riser sand layer thickness, and accordingly, to have every mold with the same total riser volume. For a typical 36 inch diameter wheel with 13 risers, the volume of metal in all risers is equal to about 32.5 percent of the weight of a finished wheel. Such volume of metal is necessary in order to have dimensional accuracy and porosity in the finished cast wheel at or below an industry standard, given the insulating properties of the resin sand lined risers and the need for the steel in such risers to remain molten for a period of time. Further, due to the amount of sand needed to line each riser, each riser has about 2.0 pounds of sand.
Such an existing method of lining risers and bottom pressure casting railroad wheels in machined graphite molds requires a volume of sand that, for a 36 inch diameter with 13 risers, equals about 130 pounds to initially fill all risers. It is desirable to decrease the amount of such sand by utilizing improved insulating material as riser liners that will allow less molten steel in each riser, but allow such molten steel to remain molten long enough to achieve desired dimensional accuracy and porosity of the cast steel wheel.
It is also desirable to eliminate the use of a resin coated sand to eliminate sand handling and resin mixing operations.
It is also desirable to reduce the amount of time needed to line the riser openings. Such lining process using the phenolic resin coated sand as described above, including heating and relining, takes about 3 minutes for each cope.